Dear David,

Once again, several Inbox Insiders came to the Email Diva's rescue on this technical question. First an explanation of IPs, courtesy of Loren McDonald, vice president of corporate communications at J.L. Halsey:

Static IPs - This just means that when a server accesses the Internet, the IP address does not change.

Shared IP - Email Service Providers (ESP) will purchase a number of static IPs and share some of them, i.e., several customers' emails will go out with the same ("pooled" or "shared") IP address. In a pooled IP environment, another company or companies' bad practices can spoil the pool party (deliverability) for everyone.

Dedicated IP - An ESP's client pays for a dedicated IP address. Since it doesn't change, the reputation belongs to a single client, rather than a group of clients.

Delving into the server issue, Joshua Baer, chief technology officer at Datran Media and Technology Chairperson with the ESPC, explains, "If you have a dedicated server, by definition you have a dedicated IP. Two different servers can't share an IP address. If you are on a shared server, you can have a dedicated IP or share one. One server can have multiple IP addresses on it. Dedicated IPs are for deliverability / reputation, while dedicated servers are for scaling large customers."

One would think that the best way to protect one's reputation and ensure optimum deliverability would be to have a dedicated, static IP. Not so says Ed Henrich, vice president of professional services at Responsys:

"Two things I'd add to the notion of a dedicated IP address and corporate accountability:

1. Some anti-spam organizations will block full class Cs* based on a single bad behaving IP, so that being on your own island doesn't fully protect you from the behavior of your 'neighbors.'

*(ISPs typically sell large customers blocks of IP address, the common unit being a class C, which is 256 addresses. They are numbered in order, so that blacklisters can block all 256 with the same level of effort as blocking just one address.)

2. Having a dedicated IP means being accountable, not safer. Most mailers don't realize that if you move from a pool to a dedicated IP, your deliverability could go down. If it does, it means you were the dirty guy in the pool."

An important first step is to determine whether you're "the dirty guy in the pool," or suffering from your neighbors' or ESP's reputations. The Email Diva recommends a delivery audit. For about half the cost of one campaign, a company like Habeas or Return Path can answer your questions, identify problems and recommend solutions.

¡Good Luck!

The Email Diva

P.S. As I was writing this article and went to Wikipedia for the millionth time, I found a solicitation for the Wikipedia Foundation.Wikipedia Founder Jimmy Wales' vision is that "every single person on the planet is given free access to the sum of all human knowledge." If you appreciate this amazing resource and would like to help spread access to developing countries around the world, please consider a donation.